A fuel cell system converts a chemical energy of a fuel directly into an electric energy. The above fuel cell system is provided with a pair of a positive electrode (anode) and a negative electrode (cathode) with an electrolyte membrane interposed therebetween. A fuel gas containing hydrogen is supplied to the anode, while an oxidizer gas containing oxygen is supplied to the cathode, thus causing an electrical chemical reaction (shown below) on electrolyte membrane sides of the respective anode and cathode. The thus caused electrical chemical reaction helps take out the electric energy from the above electrodes. Refer to Japanese Patent Application Laid-Open No. 8 (1996)-106914 (=JP8-106914).Positive electrode (anode): H2→2H++2e−Negative electrode (cathode electrode): 2H++2e−+(½)O2→H2O  (Chemical Formula 1)
Known methods of supplying the hydrogen of the fuel gas to the anode include directly supplying the hydrogen from a hydrogen storer, supplying a hydrogen-containing gas through reformation of a fuel containing hydrogen, and the like. Examples of the hydrogen storer include a high pressure gas tank, a liquefied hydrogen tank, a hydrogen-absorbing alloy tank, and the like. Examples of the fuel containing hydrogen include natural gas, methanol, gasoline and the like. On the other hand, air is commonly used for the oxidizer gas supplied to the cathode electrode.
When being used for a power source for driving an automobile or being placed in a cold place, for example, the fuel cell may be exposed to a 0° C. or less atmosphere. It is desired that the fuel cell be capable of starting even in the above state and ordinarily generating power. Under the low temperature state of 0° C. or less, however, the moisture remaining in cells of the fuel cell after the former power generation is frozen, thereby causing a problem such as a power generation failure which may be attributable to blocking of a reactive gas passage for distributing the hydrogen gas or air gas or attributable to reactive gas diffusion prevention due to the freezing of remaining moisture in the vicinity of the electrodes.
For starting the fuel cell at 0° C. or less, therefore, it is necessary to remove the moisture in advance from inside the fuel cell. Japanese Patent Application Laid-Open No. 2001-332281 (=JP2001-332281) discloses a technology of supplying into the fuel cell an un-humidified air, thereby drying inside the fuel cell to a certain humidity (dried state), to thereafter stop the fuel cell system.
According to the technology of JP2001-332281, however, drying inside the fuel cell with the reactive gas which is merely un-humidified takes a long time to accomplish a sufficient dried state for the power generation from 0° C. or less, which is problematical. Especially, when the fuel cell is used for the power source for driving the vehicle, a long time is spent until the fuel cell system stops after a driver turns off an ignition key, which is practically not preferable.
For solving the above problem, Japanese Patent Application Laid-Open No. 2002-313394 (=JP2002-313394) discloses a technology, wherein, when the fuel cell system is stopped, the fuel cell is dried with a reactive gas dried with a dehumidifier provided for removing moisture from the reactive gas by dehumidifying the reactive gas.
In addition, as a like technology, Japanese Patent Application Laid-Open No. 2002-208421 (=JP2002-208421) discloses a technology of drying a fuel cell by supplying to the fuel cell a dry air heated to a high temperature.
In addition, Japanese Patent Application Laid-Open No. 2002-246054 (=JP2002-246054) discloses a technology, wherein a coolant for cooling the fuel cell during operation is heated at the stop of the fuel cell such that the thus heated coolant heats up the fuel cell to a certain temperature, thereby drying the fuel cell.
The above technologies increase the reactive gas temperature or the fuel cell temperature, thereby evaporating and removing the moisture in the fuel cell.